Touch screen devices are prevalent in today's computing environment. Portable computers, desktop computers, tablets, smart phones, and smartwatches typically employ a touch screen to gain user input for navigation and control of these devices. Thus, discerning the intent of the user via touch inputs is an important feature of a touch screen device.
Touch screens typically operate based on capacitive touch sensing, and include a patterned array of conductive features. For instance, the patterned array of conductive features may include sets of lines, conductive pads, overlapping structures, interleaved structures, diamond structures, lattice structures, and the like. By evaluating changes in capacitance at different lines or sets of lines, a user touch or hover, such as by a finger or stylus, can be detected.
Two common capacitive touch sensing techniques or modes that may be performed on touch screens are mutual capacitance sensing and self capacitance sensing. In a self capacitance sensing mode, a drive signal is applied to every line, regardless of orientation. Bringing a finger or conductive stylus near the surface of the touch screen changes the local electric field, increasing the capacitance between the drive line or sense line of interest and ground (the “self capacitance”) in this instance. However, since all lines are driven, the capacitance change can only be measured on a per line basis as opposed to a per capacitive node basis. Therefore, the output of self capacitance sensing is two one dimensional arrays of values, with one value for each line.
As can be appreciated, the signal to noise ratio when measuring an entire line is high, and therefore self capacitance sensing allows for precise measurements. However, a primary drawback with self capacitance sensing is an inability to resolve touches by more than a single finger accurately.
In a mutual capacitance sensing mode, a drive signal is applied to a subset of the lines referred to as drive lines, and capacitance values are measured at a subset of the lines referred to as sense lines, with it being understood that the sense lines cross the drive lines in a spaced apart fashion therefrom. Each crossing of drive line and sense line forms a capacitive node. Since bringing a finger or conductive stylus near the surface of the touch screen changes the local electric field, this causes a reduction in the capacitance between the drive lines and the sense lines (the “mutual” capacitance), and the capacitance change at every individual capacitive node can be measured to accurately determine the touch location. Therefore, the output of mutual capacitance sensing is a two-dimensional matrix of values, with one value for each capacitive node (crossing between drive line and sense line). Thus, it can be appreciated that mutual capacitance sensing allows multi-touch operation where multiple fingers or styli can be accurately tracked at the same time. For this reason, mutual capacitance sensing is widely used.
However, mutual capacitance sensing is not without its own drawbacks. For example, the signal to noise ratio when measuring a single capacitive node is low. This makes noise reduction of particular interest to mutual capacitance sensing implementations.
One way to increase decrease the signal to noise ratio is to use a charge pump to supply the drive circuit that generates the drive signal. A touch screen system 10 utilizing a charge pump in this fashion is shown in FIG. 1. The touch screen system 10 includes a touch panel 12 formed by a plurality of parallel drive lines 14 and a plurality of parallel sense lines 16. The drive lines 14 and sense lines 16 are formed of a transparent material (e.g., indium tin oxide ITO) so as to not obscure a visual display system (not shown) positioned underneath the panel 12. The drive lines 14 and sense lines 16 can each be formed of a plurality of series connected diamond shapes. The drive lines 14 extend across the panel 12 with a first orientation direction (for example, horizontal) and the sense lines extend across the panel 12 with a second orientation direction (for example, vertical) such that the lines 14 cross over the lines 16 (or vice versa). However, the plane containing the lines 14 and the plane containing the lines 16 are separated from each other by a layer of dielectric material. A sense capacitor 18 is formed at each location where the lines 14 and 16 cross.
A digital controller circuit 20 generates an alternating current (AC) drive signal (VTX) in the form of a square wave, and sequentially applies that AC drive signal to the drive lines 14 through a driver circuit 22. The AC drive signal has a frequency fd that is in the range of 100-300 kHz, for example 200 kHz.
The digital controller circuit 20 is powered from a power supply voltage Vdd, with Vdd at 3.3V. The driver circuit 22 is powered from a power supply voltage Vddh, where Vddh>Vdd, with Vddh for example at 6V, 9V, 12V, 16V, or higher as needed. A charge pump circuit 24, powered from the power supply voltage Vdd, operates to boost the Vdd voltage to produce the Vddh voltage. An oscillator circuit 26 provides an AC signal 28 to the charge pump circuit 24 to control the boost switching operation of a flyback capacitor that generates the Vddh voltage. The AC signal 28 has a frequency fo that is, for example, in the range of 10-100 MHz, for example at 48 MHz.
The driver circuit 22 includes a level shifting and buffering circuit to level shift the AC drive signal output from the digital controller circuit 20 from the Vdd voltage level to the Vddh voltage level to generate the level-shifted AC drive signal (Vdrive) for application to the drive lines 14.
A charge conversion circuit 30 such as a charge to voltage (C2V) converter circuit (or a charge to current (C2I) converter circuit) is coupled to the sense lines 16. The conversion circuit 30 senses the charge at each sense capacitor 18 and converts the sensed charge to an output signal (voltage or current) indicative of the sensed charge. The amount of charge at each sense capacitor 18 is a function of the AC drive signal, the capacitance between the drive line 14 and sense line 16 at the sense capacitor 18, and the influence of a touch capacitance contributed by the presence of an object (such as a finger or stylus) in proximity to the drive lines 14 and sense lines 16 of the panel 12. A processing circuit 32 receives the output voltages from the conversion circuit 30 for each sense capacitor 18. The output voltages are processed to determine the presence (touch and/or hover) of the object and the location of the object.
While this touch screen system 10 is somewhat effective at increasing the signal to noise ratio, it has the drawback of using the AC signal 28 (from the oscillator circuit 26) fed to the charge pump circuit 24 to control the boost switching operation of the flyback capacitor that generates the Vddh voltage which is used in generating the level shifted AC drive signal Vdrive. Since the AC signal 28 is not synchronized to the level shifted AC drive signal Vdrive, the frequency of the AC signal 28 is necessarily high so as to achieve proper regulation of the level shifted AC drive signal Vdrive, leading to increased power consumption.
Therefore, an improved design was devised. Reference is now made to FIG. 2 showing an improved configuration for a touch screen system 100. The system 100 includes a touch panel 12 formed by a plurality of parallel drive lines 14 and a plurality of parallel sense lines 16. The drive lines 14 and sense lines 16 are typically formed of a transparent material (e.g., indium tin oxide ITO) so as to not obscure a visual display system (not shown) positioned underneath the panel 12. The drive lines 14 and sense lines 16 can, for example, each be formed of a plurality of series connected diamond shapes. The drive lines 14 extend across the panel 12 with a first orientation direction (for example, horizontal) and the sense lines extend across the panel 12 with a second orientation direction (for example, vertical) such that the lines 14 cross over the lines 16 (or vice versa). However, the plane containing the lines 14 and the plane containing the lines 16 are separated from each other by a layer of dielectric material. A sense capacitor 18 is accordingly formed at each location where the lines 14 and 16 cross.
A digital controller circuit 200 generates an alternating current (AC) drive signal (VTX), for example, in the form of a square wave, and sequentially applies that AC drive signal to the drive lines 14 through a driver circuit 22. The AC drive signal has a frequency fd that is, for example, in the range of 100-300 kHz and is typically at 200 kHz.
The digital controller circuit 200 is powered from a power supply voltage Vdd, with Vdd typically at 3.3V. The driver circuit 22, however, is powered from a power supply voltage Vddh, where Vddh>Vdd, with Vddh for example at 6V, 9V, 12V, 16V, or higher as needed. A charge pump circuit 204, powered from the power supply voltage Vdd, operates to boost the Vdd voltage to produce the Vddh voltage. The digital controller circuit 200 supplies an AC control signal 208 to the charge pump circuit 204 to control the boost switching operation that generates the Vddh voltage. The AC control signal 208 has a frequency fo that is, for example, the same frequency fd as the AC drive signal. The AC control signal 208 and the AC drive signal may be phase aligned.
The driver circuit 22 includes a level shifting and buffering circuit to level shift the AC drive signal output from the digital controller circuit 200 from the Vdd voltage level to the Vddh voltage level to generate the level-shifted AC drive signal (Vdrive) for application to the drive lines 14.
A conversion circuit 30 such as a charge to voltage (C2V) converter circuit (or a charge to current (C2I) converter circuit) is coupled to the sense lines 16. The conversion circuit 30 senses the charge at each sense capacitor 18 and converts the sensed charge to an output signal (voltage or current) indicative of the sensed charge. The amount of charge at each sense capacitor 18 is a function of the AC drive signal, the capacitance between the drive line 14 and sense line 16 at the sense capacitor 18, and the influence of a touch capacitance contributed by the presence of an object (such as a finger or stylus) in proximity to the drive lines 14 and sense lines 16 of the panel 12. A processing circuit 32 receives the output voltages from the conversion circuit 30 for each sense capacitor 18. The output voltages are processed to determine the presence (touch and/or hover) of the object and the location of the object.
The touch screen system 100 is configured with the charge pump circuit 204 synchronized to the application of the AC drive signal to the drive lines 14 of the panel 12 and adaptive to different capacitive loads in different modes of operation (for example, mutual-capacitance sensing or self-capacitance sensing) of the panel 12. This results in a higher efficiency of the charge pump circuit 204 and a reduction in system noise in comparison to the FIG. 1 circuit. The principle of operation with system 100 is to take advantage of the fact that the load of the charge pump circuit 204 is not a continuously resistive load (as in FIG. 1), but is instead a sample switching capacitor load. The charge pump circuit 204 is controlled for operation at a much lower operating frequency fo (that is equal to the frequency fd of the AC drive signal) resulting in an improvement in power consumption (with an efficiency of 85-90%). Additionally, the synchronized operation of the charge pump advantageously ensures that the voltage is well settled by the time the conversion circuit 30 senses the charge at the sense capacitor 18. At all other times, accurate regulation of the voltage output from the charge pump circuit 204 is not required.
While this touch screen system 100 of FIG. 2 represents a notable improvement over the touch screen system 10 of FIG. 1, improvement may still be made. As shown in FIG. 3, overshoot is generated, which prevents the signal to noise ratio from being increased as much as is theoretically possible. Therefore, despite the advances made with the touch screen system of FIG. 2, further development is possible.